1. Field of the Invention
The present invention relates to methods and means for the removal of water and the attendant lowering of the level thereof in steam generating systems and; more particularly, a new method and technique for the introduction of an inert gas, such as nitrogen, into any number of possible gas introduction points, so long as such nitrogen finds its way beneath the tubesheet and into at least one open end of each inverted U-tube thereover. said gas introduction points usually being in the lower portion of the channel heads of the type of steam generators typically used in conjunction with the operation of numerous nuclear power plants. The instant invention relates to methodologY for introducing, through any number of such introduction points, such inert gas in a manner such that it rapidly rises through the water contained in the channel head and onto the lower side of the tubesheet, which tubesheet defines a generally horizontally disposed plane comprising the upper boundary of said channel head and the terminus of said inverted U-tubes.
As noted above, during the operation of high-pressure water, nuclear-fired, power or propulsion systems there are necessitated the periodic occurrences of system outages such as when refueling is to be performed or repairs are to be effected. It is usual for the level of the primary coolant water within the system to be lowered during such outages to predetermined levels and subsequently further adjusted vertically up and down and sometimes relative to the elevation of the reactor and the level of the spent fuel holding pit, which pit normally is at an elevation higher than the fuel rods disposed within the reactor. In the type of reactor system which serves a plurality of vertical U-tube type steam generators, such draining down through the chemical volume control system and the residual heat removal system and thence into holdup tanks is conducted, of course, only after the system has been cooled and depressurized, the main circulating coolant water pumps have been shut down, the pressurizer has been depressurized, and the residual heat removal pumps have been activated to actuate the residual heat removal system.
Typically, during such drain-down procedure, a gas which is inert to the system. Such as nitrogen, is introduced into the system through the top of the system pressurizer tank to displace the water which is draining from the system. As the water is drained from the pressurizer, whose discharge flows into one of the "hot legs" or reactor outlet conduits which leads to the bottom of one of the steam generators, the nitrogen introduced through the pressurizer will eventually begin to disperse throughout the system from the bottom of said pressurizer. The nitrogen first enters the reactor above the water level therein via the reactor outlet conduit to which the pressurizer is connected, and then passes via all of the reactor inlet and outlet conduits, which are oftentimes horizontally disposed at the same elevation, to the respective steam generators and main circulating pumps in the respective steam generating loops.
As will undoubtedly become more readily apparent from a reading of the teachings of the instant invention, infra, it is inherent in the design of such systems that a total drain-down thereof would effect a lowering of the water level therein to an elevation below that desirable for the subsequent conduct of certain operations. Accordingly, it is desirable and perhaps more importantly necessary, at the appropriate stage to secure certain portions of the system against flooding during subsequent temporary raising of the water level in such system.
From an inspection of an elevational view of a system typical of the type herein referenced, it will be appreciated that the situs comprising the juxtaposition of the nozzle ring and the outlet conduit of the channel head provides a proper and convenient elevation to so secure the water level in this area of the system to ensure that maintenance work, for example, may proceed concurrently on both the primary water side and the secondary water sides thereof. A nozzle dam generally comprising a gasketed plate adapted for operative association with the nozzle ring is the usual means which is secured at such situs to seal off the opening of the nozzle ring and effect closure of the steam generator outlet conduit.
Until about the present time, the state of the art for installation of such a nozzle dam inside the confines of each half of a typical channel head, required the ingress of human operators through a relatively small aperture known as the manway for the placing, orientation, and securing of such a nozzle dam onto the nozzle ring. Since in a Westinghouse design there are usually four steam generators associated with a single reactor, and two nozzle rings in each channel head and, still further, since the secured nozzle dams must subsequently be unsecured from the nozzle rings and removed from the channel heads, there may be no less than 16 occurrences of a plurality of human operators entering and leaving an environment highly hostile to human life. Accordingly, although the main thrust of the instant invention relates to expediting the drain-down of such installations in the shortest possible time, another consideration to which the instant invention relates is the effecting of as complete an emptying of each and every inverted steam generator U-tube to ensure that none of same have a slug of water "hanging" therein such that such slug of highly radioactive water is accidentally discharged onto any of the human operators subsequently engaged in their maintenance business in the channel head, or for that matter, below or near the open manway of said channel head.
2. Description of the prior Art
As has been noted, supra, at least one prior-art investigator has taught and disclosed methods and/or means for increasing the efficiency of. and decreasing the drain-down time for nuclear steam generating systems. It is noted in the teachings of U.S. Pat. No. 4,649,019, Jawor, Mar. 10, 1987. that a principal feature of his invention requires that the nitrogen, which normally is the gas introduced in a manner so as to rise through the water in the divided channel head, effectively further rises or bubbles up into each of the inverted U-tubes thereby "releasing" the vacuum pocket formed at the top of each of such inverted U-tubes. In the practice of his invention he teaches the continuous introduction of such nitrogen and that same is maintained at a pressure of about 20 psi. Although unclear, it is assumed that this pressure should be expressed as 20 psig since he sees fit to utilize such designations or terminology throughout his teachings as it relates or refers to other portions of the steam generating system. In addition, if the designation is to psia instead of psig this would allow for only about a 5 psi differential between the system and ambient pressure, which would be rather difficult to judge or maintain. Furthermore, judging by the normal orifice size provided in situ by his most preferred embodiment, i.e., the high-impulse side of the flow transmitter, and the 16 cfm he reports to be injected therethrough, a pressure differential of about 20 psia rather than about 5 psia would appear to be correct. He also teaches that although theoretically the drain-down time of his system may be reduced to as little as 12 hours, it usually takes at least 16 hours.
The instant invention overcomes many of the difficulties inherent in operating the method of Jawor, supra, and according to the results of projected modeling, will result in a total drain-down time more closely approximating the theoretical considerations advanced, supra.
As quite correctly treated in Jawor's disclosures and teachings, the nitrogen introduced thereinto must be above atmospheric pressure in order for same to be injected into the system, and in the preferred embodiment such introduction of nitrogen is commenced at the beginning of the drain-down procedure. It will also be appreciated that as disclosed in his treatment of the prior art, once the drain-down level has reached the point generally defined as elevation E, the nitrogen introduced through the pressurizer will begin to disperse throughout the system via the reactor coolant water outlet conduits which are horizontally disposed at this same elevation, which conduits also serve as the primary water inlet conduits for each of the respective steam generators via the primary water inlet half of the channel head. At this point in the drain-down procedure, the water column, heretofore continuously maintained in the inverted U-tubes as well as in that divided channel head portion normally receiving the superheated primary water from the reactor and the intermittent leg, will be disrupted. It is at this point in Jawor's procedure wherein he concedes that the nitrogen from the pressurizer in "an expanded condition" ceases to be substantially effective, i.e., as to any water still trapped in the inverted vertical U-tubes, the nitrogen gas from the pressurizer, which is now flooding the bottom of the tubesheet, will not effectively rise thereinto and quickly or effectively release any of vacuum pockets which might still exist therein.
It has now been determined in the practice of the procedure outlined, supra, that the gauge pressure allowable for introduction of nitrogen into the system may, indeed, be rather critical. On one hand, it is important that same be maintained as high as possible to effect the advantages realized by ensuring that the gas is injected into the water column in a manner so as to ensure that the bubbles formed therein are not carried away by water flowing through the downwardly directed intermittent legs before such bubbles are allowed to rise to the bottom of the tubesheet, spread out across same, and rise within each of the several thousand tubes therewith juxtaposed. In addition, high inlet pressure for such nitrogen introduction will ensure sufficient gas in each of the inverted U-tubes for displacement of the volume of the water drained therefrom so as not to induce, or allow for formation of, a vacuum or partial vacuum therein. On the other hand, it has now been discovered that if too large a volume of nitrogen is so introduced, by virtue of such high inlet pressure, a gas-liquid interface is formed across the bottom of said tubesheet in a manner so as to disrupt the water column which was previously continuous above, through, and below this position of such now formed gas-liquid interface thereby interfering with the very objectives of Jawor's teachings. Jawor teaches, that when his drain-down has progressed to the point wherein the water level has reached an elevation common with the tops of the hot and the cold legs and represented generally at, or perhaps more concisely just above, E, the nitrogen which is introduced through the pressurizer enters and spreads horizontally throughout the system via such conduits, enters the respective channel heads, and allegedly begins to bubble upwardly into the tube bundles. Contrary to these teachings, my modeling of this configuration indicates that at this time the water level in at least that half of the channel head into which he introduces nitrogen has receded to the same, or substantially the same elevation E. Accordingly, the description, supra, that the nitrogen begins to bubble upward may not only be less than accurate, it may lead those skilled in this art away from the actual occurrence of events, and as will become more apparent from the description, infra, away from the teachings of the instant invention. For instance, it is herein suggested that as the drain-down procedure progresses through its initial stages, the nitrogen introduced according to the teachings of Jawor, supra, does bubble up through the water column then present in the channel head to the tube face and thence into the open bottom ends of the inverted U-tubes comprising the primary water side of the steam generators. However, once the level of the water in at least that divided portion of the channel head, in tracking the level in the rest of the system, is lowered below the bottom of the tubesheet, any resulting formed gas-liquid interface effectively and substantially blocks the further transfer of nitrogen bubbling up through the water still in the bulk of the channel head below such formed gas-liquid interface and any water still trapped in the tubes thereabove. It is at this point in the drain-down procedure of Jawor wherein a substantial loss of the effectiveness thereof occurs if, up to this time, the volume of nitrogen introduced thereby has not been perfectly balanced with the volume of water which has been drained from said inverted U-tubes. More importantly, it has now been discovered that given the operating parameters set forth and suggested by Jawor, such a balance of volume of nitrogen introduced into the divided channel head with the volume of water drained from said tubes does not and, indeed, cannot occur. Accordingly, it is precisely because of this inherent imbalance of volumes, or in another vernacular a ratio of volume of nitrogen introduced into the divided channel head to the volume of water removed from the inverted U-tubes thereabove of greater than 1:1, that the gist underlying the principal concept of the instant invention depends. It is noted that there are numerous references throughout these teachings wherein reference is made to volumes or volumetric quantities of gas, including nitrogen. For the sake of convenience to the reader, it is to be understood that comparisons therebetween, as for instance in expressed ratios thereof, are made at or under the same or equivalent conditions of temperature and pressure, unless otherwise indicated. With the above considerations in mind, it will now be appreciated that when such ratio, supra. exceeds 1:1, an excess of nitrogen will accumulate at the top of the water in the channel head and below the bottom of the tubesheet thereby forming the instantly discovered as well as the undesirable gas-liquid interface long before the water level in the system is lowered to the elevation of said tubesheet. As described above, when such an interface is formed, the water remaining in the tubes will effectively be trapped therein. Indeed, in the operation of several such nuclear-powered generating units, by employees of the assignee of the instant invention, it has been observed during such drain-down procedure that water entrapment can result in "gurgling" from the open ends of the U-tubes in such a system for several days and even upwards for almost a week after the initiation of drain-down.
With reference to the inherent imbalance of the volumes in and out of the channel head and U-tubes, respectively, it may be seen from the teachings of Jawor, supra, that his input of nitrogen at the rate of 16 cfm would completely fill the tube bundle in as little as about 45 minutes. Since the minimum time ascribed for the best attainable drain-down is about 16 hours, it will be appreciated that he teaches an input of nitrogen which exceeds any possible outflow of entrapped water by at least one order of magnitude. Accordingly, such prior-art teaching would inherently result in a ratio of volume of nitrogen introduced into the divided channel head to the volume of water removed from the inverted U-tubes thereabove of greater than about 20:1, which, of course, is way in excess of the maximum of 1:1 herein taught as being critical to the effecting of the instant invention and necessary for attaining the principal objects thereof.